The present invention relates to an improvement in a process for preparing a tinted contact lens. In particular, the invention concerns contact lenses comprising polymeric lens material in which reactive dyestuffs have been covalently bonded to monomer units of the polymer backbone. This invention is of particular interest in the area of so-called hydrophilic or "soft" contact lenses, commonly referred to as hydrogel lenses.
There are many known methods for tinting or coloring contact lenses. For example, a coloring agent, such as dyestuff or pigment, may be dispersed in a monomer precursor or polymer matrix. This method is typically unsuitable for use in hydrophilic contact lenses, however, because the coloring agent leaches from the plastic upon presence of water in the lens.
Another known method of coloring lenses is printing or otherwise applying the coloring agent to the lens surface. This method is also problematic, because a portion of the agent often becomes dissociated from the lens when the lens is placed in water. The primary cause of this problem is insufficient physical adhesion of the coloring agent to the surface. To counteract this problem, it is necessary to use a large amount of agent in the coloring process.